1. Field of the Invention
The present invention relates to a device and method for cutting hard and soft tissue and in particular, teeth, tooth enamel, tooth dentin, tooth cementum, bone, cartilage, skin, mucosa, gingiva, and muscle, and preferably in the oral cavity.
2. Background of Related Art
The use of lasers for oral applications was reported as early as 1964. While such reports indicated that lasers could be used on dental hard tissue, lasers have not generally been used clinically until recently for surgical purposes, including drilling or cutting of teeth, primarily because of the large amount of damage caused to the material and its surrounding tissue by the laser. Such damage is generally caused by excessive heating of the surgical site and its surrounding material due to the amount of energy required by the laser to cut the material. However, recent developments with the use of pulsed eximer and infrared lasers on soft and hard tissue has shown promise, resulting in cutting of these materials with less damage then previous lasers.
Infrared Erbium lasers are suitable for cutting soft and hard biological tissue. This is because of the high absorption that biological tissues have for laser energy at the wavelength generated by these lasers, 2.7 xcexcm to 2.94 xcexcm. This high level of absorption is due largely to the high water content of these materials since water has an absorption coefficient of about 10,000 to 13,000 per centimeter at these wavelengths. As a result, many researchers have attempted to improve to the cutting ability of Erbium lasers by adding of externally applied high energy absorptive fluids, such as water, primarily for the purpose of cooling the material being cut and its surrounding tissue. The disclosure of the following patents are incorporated into this application by reference.
In Vassiliadis et al., U.S. Pat. Nos. 5,257,935, and 5,342,198, the application of water is disclosed, not for use as a cutting aid but for cooling the lazed material, if necessary. Both Vassiliadis patents teach the use of a laser to eradicate, by vaporization, tooth material by exposing the tooth to the laser radiation followed by a water spray if heating does occur. While the patent states that the apparatus obliterates dental enamel and dentin without significant heating, it is known that the heating of tooth material to facilitate obliteration must be conducted at ablation rates that are somewhat less then optimal or else unwanted charring and discoloration of the tooth material will occur. Additionally, vaporization of tooth material requires quite high temperatures which in turn promotes tissue and tooth damage.
U.S. Pat. No. 5,401,171, to Paghdiwala, discloses the use of a handheld tool containing a built in laser and delivery system which focuses the laser energy on a surface to be cut. Paghdiwala additionally discloses the use of externally applied water to cool the surface being cut, thereby preventing excessive heating.
Allen, U.S. Pat. No. 4,987,286, discloses the use of water illuminated by laser light to remove particles adhered to a surface without the danger of damaging the underlying surface. In Allen, the process removes particles and contaminates adhered to the surface of a base material by laser energy which excites a fluid, typically water, surrounding the particles. The laser causes the water to explosively evaporate with sufficient force to remove the attached particles, but without cutting into or causing damage to the base surface material.
Steiner et al., U.S. Pat. No. 5,199,870 teaches the use of a thin layer of absorbing fluid, on the order of 10 to 200 microns, applied to the surface of the tooth prior to the application of laser light. They claim that Laser light heats the liquid, causing the liquid to expand and cool the tissue surrounding the cutting location.
Wolbarsht et al., U.S. Pat. No. 5,267,856 teaches the use of a fluid which is applied to a material prior to or during a laser pulse, permitting the fluid to enter into pores and cracks of the material, instead of remaining pooled on the surface. The invention requires that the water, or whatever liquid or radiation absorbent substance used, must infiltrate into the pores of the material without forming a barrier to penetration of the laser radiation into the material surface. The pooled fluid can be removed by moist air before the laser light is applied; dry air should not be used to blow of the excess water unless care is taken to be sure water remains in the pores. The ablation of the material by this process results from the expansion of the fluid seeped into the pores. However, since many materials such as tooth enamel are not very porous, and since other human hard tissue already has high levels of internal water present in the tissue, the penetration rate of such liquids into living tissue is very low, limiting the effectiveness of this cutting method.
Biolase Technology, Inc. PCT Patent Application No. PCT/US96/13960 teaches a process in which a mist of atomized particles is introduced into a volume of air above a target surface to be cut. A laser is focused into the mist causing the atomized particles to explode and impart mechanical cutting forces to the target surface. Accordingly, the exploding fluid particles rather then the direct laser energy source cuts the tissue; the laser energy itself is not directly absorbed by the target material. While this method may provide some level of ablation of the target material, it is not very efficient because (1) the water absorbs the laser energy and the heat of vaporization which is lost and can not be converted into cutting energy, and (2) the particles accelerated by the explosion of the water bounce off the target tissue due to the high acoustic impedance of the target surface, imparting little energy to the target material.
The use of an absorbing fluid in Steiner and Wolbarsht et al. has inherent efficiency limitations. Ideally, one would want a synergistic effect to be generated by the laser in directly ablating a surface material and interacting with the applied fluid to further enhance the ablation process. However, with the above processes this is not possible because the fluid has a great affinity for the laser energy causing the laser to first interact with the fluid and convert it to a gas. All the energy used to heat the fluid to a gas (latent heat of vaporization) is lost and not available for the ablation process. This conversion takes place during the initial portion of the laser pulse, and prior to actual ablation of the material surface. The fluid is thus blown away by gaseous expansion prior to the ablation and is therefore is not available during the remainder of the laser pulse. In addition, the use of a fluid alone, such as water, is insufficient in many materials to generate ablation forces since the force produced by the fluid gasification pressure is not high enough to fracture the material. Thus, the use of an absorbing fluid may actually lower the efficiency of the cutting system, resulting in inefficient use of the laser energy with less than optimum performance results.
The process of cutting by vaporization, as disclosed in Vassiliadis, also has inherent efficiency limitations. The cutting of tooth material by vaporization required quite high temperatures, typically on the order of 2,000 C. Thus cutting must be conduced quite slowly in order to prevent heating and damage to the tooth and surrounding tissue. If higher cutting rates are attempted, charring or burning of the tooth and surrounding tissue can occur as well as permanent damage to live tooth and tissue structure.
The present invention involves a device and method for ablating hard and soft tissue and, in particular, teeth, tooth enamel, tooth dentin, tooth cementum, bone, cartilage, skin, mucosa, gingiva, and muscle, and preferably in the oral cavity. This is accomplished by interacting an electromagnetic energy source, typically a laser, with a dry tissue surface to heat the surface to a temperature at which thermo-elastic failure (thermal expansion induced stresses) of the material takes place due to rapid heating. This heating results in the xe2x80x9ccrumblingxe2x80x9d or xe2x80x9csnappingxe2x80x9d of the tissue rather then vaporization. When this point is reached, the laser energy is stopped and a cool fluid is immediately applied to the ablation site to create additional thermal stresses, resulting in further ablation of tissue by the thermal expansion stress of rapid cooling. In addition, the fluid is delivered to the ablation site with sufficient momentum so that it not only quickly cools the site but also scrubs and flushes the surface of debris (because the process does not vaporize the material), flushing particles off of the surface and out of the lasers field of view. The momentum of the fluid flow is also high enough so that all or most of the fluid delivered to the ablation site bounces off, leaving the surface relatively free of the fluid and without the fluid seeping into the material""s pores or cracks.
The present invention also addresses a method and apparatus for a laser delivery system which reduces the absorption of ablated debris. This is accomplished by directing the fluid so that it not only forcibly remove debris from the ablation site, but also interacts with the tip of the laser delivery source to keep it free of contaminates. The fluid utilized may be gaseous or liquid and is an efficient solvent for flushing the target material. The fluid is preferably non-absorbent of the lasing energy having heat capacity and latent heat of vaporization.